A virtual machine emulates an entire computer, including the hardware and the operating system. Virtual machines are resource-hungry - they require large amounts of RAM and hard disk space, since they must run an entire operating system.

A container is a more lightweight solution. Containers ideally contain a software application complete with the libraries it needs to perform its task. However, it takes skill and discipline to build a container that simple, and many containers are built on the entire root filesystem of the underlying operating system, as well as including build-time dependencies. This can make them quite large.

<aside> 💡

This performance gain is only true when running on Linux: if it's running on MacOS, the container has to run in Docker's environment which is effectively a virtual machine. For Windows, Docker must run on WSL (Windows System for Linux), which comes with its own overheads and issues. It uses Windows Hyper-V, so again, effectively, it’s using a virtual machine.

</aside>